The invention concerns a device with electrodes for the formation of a ball at the end of a wire.
Such devices are used on so-called Wire Bonders. A Wire Bonder is a machine with which semiconductor chips are wired after mounting on a substrate. The Wire Bonder has a capillary which is clamped to the tip of a horn. The capillary serves to secure the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On producing the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding out of the capillary is first melted into a ball. Afterwards, the wire ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonics. In doing so, ultrasonics is applied to the horn from an ultrasonic transducer. This process is called ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the substrate. This last part of the process is called wedge bonding. After securing the wire to the connection point on the substrate, the wire is torn off and the next bonding cycle can begin.
In order to form the end of the wire protruding out of the capillary into a ball, a high DC voltage is applied between the wire and the electrode so that an electrical spark occurs which melts the wire. The voltage at which the electrical breakdown of the ionised air takes place between the wire and the electrode and the spark is created is designated as sparking voltage.
Today, three types of devices are known for the formation of a wire ball at the end of a wire which prevail on the market. These three types are explained based on FIGS. 1 to 3. With the first type (FIG. 1), a flat electrode 1 is swivelled from the side under the capillary 3 which guides the wire 2. With this arrangement of electrode and wire, the electrical spark 4 is formed in the longitudinal direction of the wire. The electrical spark 4 therefore runs symmetrically to the wire. The advantage of this type is that the symmetry of the formed ball is comparatively high. On the other hand, the disadvantage is that the swivelling in and out of the electrode costs time. Furthermore, the swivelling in and out of the electrode can stimulate oscillations of the bondhead.
With the second type (FIG. 2), an electrode I with a tip is arranged laterally offset underneath the capillary. With this arrangement, the electrical spark 4 runs non-symmetrically to the longitudinal axis of the wire which tends to lead to the formation of asymmetrical wire balls. However, the asymmetry of these wire balls has a predominant direction. This makes it possible to reduce the asymmetry with additional measures. In addition, this arrangement requires a higher sparking voltage in comparison to the first type. With the same vertical distance to the downholder plate which is located immediately below the device and holds the connection fingers on the substrate in position, this leads to a greater occurrence of electrical discharges on the downholder plate.
With the third type (FIG. 3), the electrode 1 is a rotationally symmetrical ring electrode. After formation of the wire ball, the capillary 3 is lowered down through the electrode. The advantage of this arrangement is that the voltage necessary for creating the electrical spark is lower than with the first two types. The disadvantage is that the place where the electrical spark 4 is created constantly changes. Wire balls formed with this arrangement also have a tendency towards asymmetries, however these asymmetries have no predominant direction.
A further device for the formation of a wire ball is known from U.S. Pat. No. 5,263,631. This device has three pointed electrodes which are separately electrically controlled in order to regulate the currents flowing through the individual electrodes. A similar device is known from U.S. Pat. No. 4,594,493. These devices produce simultaneously either three or four sparks. However, the positioning of the electrodes relative to each other is a complicated task because it must be ensured that really three or four sparks are generated.
Further devices for the formation of a wire ball are known from U.S. Pat. Nos. 4,909,427 and 5,037,023. U.S. Pat. No. 5,037,023 shows the use of different electrodes where a first electrode is used to form a wire ball and a second pair of electrodes is used to locally melt a portion of the isolation of the wire. The first electrode and the second pair of electrodes are not used in combination for the formation of the wire ball and they are sequently brought into their working positions to perform their task separately.
The object of the invention is to develop a device for the formation of a ball at the end of a wire which is distinguished by as low a sparking voltage as possible and with which wire balls can be produced the size of which varies as little as possible.
A device in accordance with the invention has a first electrode which has a tip and at least one additional electrode. All electrodes are mechanically rigidly connected with each other. The second electrode, or the further electrodes, influence the progression of the electrical field that is created between the end of the wire and the electrodes. All electrodes are connected together electrically and therefore lie on the same electrical potential. Furthermore, they are formed in such a way and arranged so that in any case the distance between the tip of the first electrode and the wire is less than the distance between the second electrode, or all further electrodes, and the wire. In this way it is ensured that only one single electrical spark is produced, namely between the wire and the tip of the first electrode. The advantage of the at least one additional electrode lies in that a clearly lower sparking voltage results than with prior art. As a result, the distance between the electrodes and the downholder plate can be reduced without increasing the risk of discharges. Therefore, for the formation of the wire ball, the capillary does not have to be raised so far so that the cycle time is reduced.
The at least one additional electrode can be open, ie, can have an open end, with or without tip. However, it is of advantage when the at least one additional electrode is a closed electrode, for example a ring-shaped electrode. A closed electrode has the advantage that the dispersion of the sparking voltage is less than with an open electrode which leads to less variation in the size of the wire ball.
In the following, embodiments of the invention are explained in more detail based on the drawing. The illustrations are not drawn to scale.